There are various methods of molding composites. One method involves matched metal molds in which two halves of a mold are pressed together and heated (cured) to form an article. Although this can be a very accurate method, the cost of manufacturing the matched metal molds is very high, making the process uneconomical for small production runs. In addition, slight inaccuracies in the layup can cause large variations in the properties of the cured composite. Another method which has had widespread use is vacuum bag molding. A thin film, typically nylon, is placed over the article to be molded, forming a bag type enclosure, and a vacuum is drawn on the bag prior to heating the assembly to cure the composite material. For high quality material, the assembly is placed in an autoclave and external pressure is applied during the cure. While the vacuum bag molding method is lower in cost and more tolerant to material variations or layup inaccuracies than the matched metal molding method, the bags tend to wrinkle and cause the molded article to have an uneven surface. Also, the vacuum bag can catch on high points in the layup and bridge, or fail to contact the entire surface. Bridging results in either improper compaction or stretching of the bag beyond its yield point with resultant failure of the bag. In either case, the properties of the cured material are degraded. Finally, there is a great deal of labor involved in placing the bags over the composite articles and sealing them, and the bags can only be used once.
Another type of vacuum bag is made from silicone rubber, either reinforced, or unreinforced. This bag is designed to be reusable, however, the silicone materials tend to interact with the curing agents used in the composites and they become brittle. This results in relatively short lifetimes.
Accordingly, there has been an ongoing search in the art for a reusable tooling material which is flexible enough to conform to the part surface and provide uniform cure pressure, and yet sturdy enough to withstand the rigors of material handling and chemical interaction with the curing resin.
In forming integral complex composite structures, which require co-curing of previously produced detailed structures such as structural supports or struts, problems occur in locating the details on the composite sheets and in preventing movement during compaction and curing. Generally, a composite material, usually a preimpregnated fiber fabric or tape in sheet form, is layed-up in a female mold and the details positioned on the sheet material by hand. The details may require physical structures, such as blocks and bridges, adhesives or other means to hold the details in place. A flexible bag is then carefully laid over the mold and drawn down with vacuum to compact and hold the plies for molding. It is very difficult to locate the details, requiring templates and hand measurements to assure proper positioning. It is also difficult to prevent movement during the addition of the bag and handling of the mold, resulting in generally unacceptable part-to-part reproducibility. Consequently, details are usually attached to the article after molding using skin penetrating fasteners. Therefore, the search continues for a method for molding highly complex integral parts with precisely located detail structures.